The present invention relates to a method of starting open-loop vector control in a synchronous machine, the method comprising the steps of determining a stator inductance model of the synchronous machine and measuring the stator inductance in a plurality of directions.
Vector control refers to a manner of controlling an AC motor which allows flux linkage and torque of the motor to be controlled independently like in a DC motor. In the DC motor, direct currents influencing the flux linkage and the torque are controlled, while in the AC motor both the amplitude and the phase angle of the currents have to be controlled. Thus, current vectors are controlled, from which comes the term vector control.
To implement vector control, the flux linkage and the current of the motor have to be known. The flux linkage of the motor is generated by the action of stator and rotor currents in the inductances of the machine. In an asynchronous machine, the rotor current has to be estimated and the estimation requires information on rotation speed of the rotor. This requires measured or estimated rotation speed of the rotor. In the synchronous machine, a magnetization current independent of stator magnetization is applied to the rotor, or the rotor magnetization is implemented with permanent magnets and its influence, seen from the stator, shows in the direction of the rotor position angle. To know the flux linkage caused by the position angle, the position angle of the rotor has to be measured or estimated.
When vector control of the AC motor employs a measured rotation speed or position angle of the rotor, the control method is known as the closed-loop vector control. If the rotation speed or the position angle is estimated, the control method is known as the open-loop vector control. Depending on the implementation method, a variable to be estimated can also be the stator flux linkage, apart from the rotor angle or angular speed.
When the synchronous machine is started by vector control, the machine""s stator flux linkage has the initial value "psgr"s0, which is dependent on the rotor magnetization "psgr"f and the rotor position angle xcex8r as follows:
"psgr"s0="psgr"fejxcex8r.
When voltage us influences the stator flux linkage, the stator flux linkage changes in accordance with the equation       ψ    s    =            ψ      s0        +                  ∫                  t          1                          t          2                    ⁢                        (                                    u              s                        -                                          R                s                            ⁢                              i                s                                              )                ⁢                              ⅆ            t                    .                    
It appears from the equation that when integrating the stator flux linkage a previous value of the stator flux linkage is required, apart from the voltage and current values. Thus, to start the machine controllably, information on the initial position angle of the rotor is required. When employing the closed-loop vector control, the initial angle is measured, whereas when employing the open-loop vector control, the initial angle has to be defined by estimation. When the rotor rotates, the rotor flux linkage generates an electromotive force which can be utilized in vector control in a normal operating situation, but at a rotor standstill there is no electromotive force.
In a salient pole synchronous machine, such as a separately excited synchronous machine or one with permanent magnet magnetization or in a synchronous reluctance machine, the stator inductance Ls in stationary coordinates varies as a function of the rotor angle xcex8r as presented in the following equation
Ls=Ls0+Ls2 cos2 xcex8r,
and FIG. 1 shows a graphic illustration of the equation. It appears from the figure that the inductance varies around the basic value Ls0 at twice the rotor angle in a magnitude indicated by the inductance coefficient Ls2. The inductance coefficients Ls0 and Ls2 are defined as follows             L      s0        =                            L          sd                +                  L          sq                    2        ,      
    ⁢            L      s2        =                            L          sd                -                  L          sq                    2        ,
where the inductances Ls0 and Lsq are the direct-axis and quadrature-axis transient inductances of the synchronous machine.
To utilize the above equation for defining the initial angle of the rotor is known per se and it is set forth, for instance, in the articles by S. xc3x96stlund and M. Brokemper xe2x80x9cSensorless rotor-position detection from zero to rated speed for an integrated PM synchronous motor drivexe2x80x9d, IEEE Transactions on Industry Applications, vol. 32, pp. 1158-1165, September/October 1996, and by M. Schroedl xe2x80x9cOperation of the permanent magnet synchronous machine without a mechanical sensorxe2x80x9d, Int. Conf. On Power Electronics and Variable Speed Drives, pp. 51-55, 1990.
According to the article by M. Leksell, L. Harnefors and H.-P. Nee xe2x80x9cMachine design considerations for sensorless control of PM motorsxe2x80x9d, Proceedings of the International Conference on Electrical Machines ICEM""98, pp. 619-624,1998, sinusoidally altering voltage is supplied to a stator in the assumed direct-axis direction of the rotor. If this results in a quadrature-axis current in the assumed rotor coordinates, the assumed rotor coordinates are corrected such that the quadrature-axis current disappears. The reference states that a switching frequency of the frequency converter supplying the synchronous machine should be at least ten times the frequency of supply voltage. Thus, the supply voltage maximum frequency of a frequency converter capable of 5 to 10 kHz switching frequency, for instance, is between 500 and 1000 Hz. This is sufficient for an algorithm to function. Switching frequencies as high as this are achieved by IGBT frequency converters, but frequency converters with GTO or IGCT power switches, required at higher powers, have the maximum switching frequency of less than 1 kHz. The maximum frequency of the supply voltage in the initial angle estimation remains then below 100 Hz. At such a low frequency the machine develops torque and the algorithm becomes considerably less accurate.
In the reference by M. Schroedl, 1990, the initial angle is calculated directly from one inductance measurement, or, if more measurements are employed, the additional information is utilized by eliminating the inductance parameters. A drawback with the method is that an error, which is inevitable in measuring, has a great influence. One example of actual inductance measurement with a permanent magnent machine at rotor angles xcex8r=[0, . . . ,2xcfx80] is shown in FIG. 2. The figure shows theoretically great deviations from the sine curve. The inductance measurement is effected such that a stator is fed with a current impulse which causes flux linkage on the basis of which the inductance is calculated. Errors may arise from an error in current measurement or from the fact that the measuring current produces torque that swings the rotor.
From the inductance expression in the stationary coordinates it is possible to derive an expression for a rotor angle             θ      r        =                            1          2                ⁢        arccos        ⁢                                            L              s                        -                          L              s0                                            L            s2                              +              n        ⁢                  xe2x80x83                ⁢        π              ,
where n is an integer. The influence of the error in the measured Ls can be studied by differentiating xcex8r with respect to Ls:             ⅆ              θ        r                    ⅆ              L        s              =            -              1                  2          ⁢                      L            s2                                ⁢          xe2x80x83        ⁢                  1                              1            -                                          (                                                                            L                      s                                        -                                          L                      s0                                                                            L                    s2                                                  )                            2                                          .      
This allows calculating an error estimate for the angle       Δ    ⁢          xe2x80x83        ⁢          θ      r        ≈                    ⅆ                  θ          r                            ⅆ                  L          s                      ⁢    Δ    ⁢          xe2x80x83        ⁢                  L        s            .      
It is observed that             Δ      ⁢              xe2x80x83            ⁢              θ        r              →    ∞    ,      xe2x80x83    ⁢            when      ⁢              xe2x80x83            ⁢                                    L            s                    -                      L            s0                                    L          s2                      →    1.  
It is observed from the above that, when the inductance difference between the direct-axis and quadrature-axis directions is small, the error estimate of the angle approaches infinite. In other words, initial rotor angle definition based on inductance measurings becomes the more unreliable, the closer to one another the magnitudes of the direct-axis and quadrature-axis inductances of the rotor.
In the method presented in the reference by S. xc3x96stlund and M. Brokemper the rotor angle is not calculated directly, but the minimum inductance is searched by starting the measuring of inductances in different directions first at long intervals and when approaching the minimum by reducing the angular difference of successive measurings. Even though it is not mentioned in the article, the method easily catches fictitious minima resulting from measuring errors, and therefore, an error value may be extremely high.
On the basis of the above, the influence of the inductance measuring errors should be reduced somehow. One method could be to employ several measurings in each direction and to calculate the average from the measured inductances, yet this procedure does not eliminate the influence of a systematic error.
The object of the present invention is to provide a method which avoids the above-mentioned disadvantages and enables starting of an open-loop vector control in a synchronous machine in a reliable manner. This is achieved by the method of the invention which is characterized by comprising the steps of arranging measured stator inductances in a determined stator inductance model in order to form model parameters giving the minimum error, checking magnetization polarity of a rotor in order to verify the direction of the rotor magnetization, initializing flux linkages of the open-loop vector control according to the formed model parameters and the direction of the rotor magnetization, and starting the synchronous machine by the vector control method.
The method of the invention is based on the idea that the magnitude of the stator inductance is measured in a plurality of directions and the inductance values obtained as measurement results are arranged in the inductance model of the machine. As a result of the arrangement, very accurate information on the initial rotor angle of the synchronous machine is obtained. In addition, by utilizing the method of the invention, information on the initial value of the rotor magnetization in the stationary coordinates is obtained, and consequently the machine can be started in a reliable manner without transients or jerking startup.